BASEMENT-COVER CONTACT RELATIONSHIPS BETWEEN MESOPROTEROZOIC ROCKS AND THE NEOPROTEROZOIC MOUNT ROGERS FORMATION, SOUTHERN APPALACHIAN BLUE RIDGE

The Neoproterozoic Mount Rogers Formation (MRF) in the Blue Ridge of SW Virginia and NW North Carolina represents an early, aborted stage of continental rifting on the eastern margin of Rodinia. The upper MRF is dominated by voluminous rhyolite lavas and ash-flow sheets, dated at ~760 Ma, whereas the lower MRF contains a bimodal basalt-rhyolite sequence overlain by immature coarse clastic deposits. The lower MRF is in contact with Mesoproterozoic meta-igneous rocks ranging in age from ca. 1330 to 1060 Ma, formed by magmatic and metamorphic events during the Grenville orogeny. The basement-cover contact is not well exposed, but it appears that all of the different lithologies of the lower MRF are, in one place or another, in contact with the basement rocks. The contact has been interpreted as a nonconformity, upon which deposition occurred on an eroded basement surface of considerable topographic relief.

We have undertaken detailed mapping to delineate the stratigraphy and structure of the lower MRF, and most recently focused on the nature of the basement-cover contact. Despite chlorite-grade metamorphism and deformation due to Paleozoic (Alleghanian?) tectonic events, identification of protoliths in the lower MRF is generally straightforward. We interpret the sequence as comprising porphyritic metarhyolite overlain by metabasalt flows, capped by arkosic metasandstone grading into coarse polymict metaconglomerate. The clastic sequence represents progradational alluvial fan deposits likely formed in active rift basins. As noted by previous workers, the conglomerate contains abundant granitoid lithic clasts that obviously were derived locally from the basement rocks; therefore the contact must have originated as an unconformity. However, our mapping indicates that zones of intense shearing and mylonitization affected both the lower MRF lithologies and the underlying basement rocks. Although high strain zones are present throughout the region, the strain is especially prominent along the contact. As this is a regional feature, we suggest that the nonconformable contact became a localized zone of high strain during Paleozoic deformation, perhaps due to competency contrast between the massive granitoids and the weaker volcanic lithologies.